On the subject of stem cells, the gold standard is creating desired tissues or organs. Everyone knows, of course, of their potential to regenerate any tissue from scratch. Converting common knowledge into practicality, however, is the domain of engineers. An obstacle to stem cell therapy is creating an environment in the body where these fragile cells can grow protected from attack by the immune system. A novel approach to solving the problem, reported by Samuel Stupp and colleagues in the March 28 issue of Science, is a quasi-biological ‘sac’ created out of hyaluronic acid (a common natural material composing synovial fluid) and small peptide molecules. The resulting polymer, a moldable, stretchable, repairable and even biodegradable, enclosure where stem cells can grow and differentiate for up to four weeks, may well be the answer. Importantly, the membranous nature of the sacs permits exchange of large proteins across it and researchers expect other large materials, including genes, to be mobile across its walls, creating a microscopic laboratory environment inside which the stem cells can be easily manipulated. Even when materials need to be introduced through holes poked in the structure by needles, the damage is readily repairable by adding more of the starting solution to the tear. The self-assembling molecular technology may also have other applications that are not purely biological. Researchers speculate it may be applicable to solar cell construction and making columns of catalytic nanostructures.